1. Field of the Invention
This invention relates to a metal blind-riveting assembly comprising a rivet having a tubular body portion with a head flange at one end and a mandrel having a stem with a neck of reduced cross section at which the stem will break at a predetermined tension, the mandrel being upset at both ends, one of which forms a rivet-setting head while the other is a pulling head for engagement by pulling means of a rivet-setting tool.
2. Statement Of The Prior Art
A blind-riveting assembly of this general type, and its production method, are shown in U.S. Pat. No. 2,317,360. In this latter assembly there are two upset ends of the mandrel which are cylindrical, or substantially so, which with their greater diameters, are shouldered in relation to the part of the stem lying betwen them. The terminal end of each of the two heads remote from the mandrel stem is defined by a flat radial surface normal to the axis of the mandrel. The head used as the pulling head of the mandrel also meets the mandrel stem at an annular face normal to the mandrel axis, while the head forming the rivet-setting head is conical at its underside where it joins the mandrel stem and forms an angle of about 45.degree. with the mandrel axis. The cone is intended to widen the body portion of the rivet when the rivet is being set. The pulling head of the mandrel is thus rectangular, or substantially so, in axial section, while at the opposite end of the mandrel, there is a corresponding profile but with the exception that here the transition into the mandrel stem is conically formed. U.S. Pat. No. 2,183, 543 and 2,538,623 also illustrate other types of rivet assemblies.
With the method described in those specifications for producing the pulling head of the mandrel, with its substantially rectangular profile, a considerable amount of pressure is necessary in order to make the material blank yield to form this profile shape, and the reaction to this pressure is taken at least in part by an abutment engaging the other end of the mandrel. Thus, in the course of this heading operation the predetermined breaking point or neck of the mandrel, where the cross section of the mandrel stem is reduced, as shown in FIGS. 8 and 9 of U.S. Pat. No. 2,183,543 and in FIGS. 1 and 3 of U.S. Pat. No. 2,538,623, is subject to a considerable load. If the neck is one in which there is a reduction of about 30% in the cross section, as known and commonly used in the art, then there is a risk that the parts of the mandrel meeting at the neck will shift against each other in such a way, in the event of a particularly high pressure in the upsetting process, that they will no longer lie axially aligned if the mandrel is not tightly enclosed at either side of the neck by a tool which prevents any lateral yielding of the parts of the mandrel at these localities. Over and above this, high axial pressures exerted on the neck may lead to such a compressive strain on it that its predetermined breaking force changes, which would deprive the blind-riveting assembly in question of its required properties and under certain circumstances would even prevent proper execution of the rivet-setting process.
The properties describecd above, on the part of the known blind-riveting assembly with a mandrel upset at both ends essentially like flat-topped cylinders, no doubt contributed to the situation whereby such a blind-riveting assembly has not become widely established in practice. The blind-riveting assembly predominantly used in practice consists of a smooth continuous nail-like mandrel with a predetermined breaking point or neck inside the rivet sleeve and a rivet-setting head which is generally conical or hemispherical in axial section terminating at a flat annular base, and no pulling head.
In other blind-riveting assemblies known in the art, the mandrel projects beyond the rivet flange in the usual way over a length which represents a multiple of the length of the rivet, and runs to a slender point. Behind this there is a bead formed on the mandrel, behing which the setting tool grips the stem and which partically absorbs the tensile forces exerted on the mandrel by the setting tool. In front of the bead, in the direction of the rivet head flange, the mandrel is roughened so as to increase the friction between its surface and the jaws of the setting tool, with the result that the tensile force exerted by the setting tool is absorbed firstly by the mandrel stem via its friction-increasing surface and secondly via the bead. According to this type of rivet assembly, the bead fullfils only the function of an additional safeguard, and does not absorb the full tensile force of the setting tool all on its own. The bead formed adjacent the pointed end of the mandrel makes it impossible to push the mandrel with the bead through the rivet when producing the blind-riveting assembly. The bead must therefore be initially shaped subsequent to the insertion of the mandel in the rivet, which is technically difficult, particularly in view of the mandel's pointed tip extending beyond the bead. In practice, therefore, the metal blind-riveting assemblies have been used with a smooth nail-like mandrel, upset at only one end to provide the rivet setting head, in which the length of the part projecting out of the rivet head flange represents a multiple of the length of the rivet, in order to provide the pulling jaws of the setting tool with a sufficiently long application surface for transmitting the tensile force.
In addition to the above, there are also known solutions to the problem of applying the tensile force, which amount to providing ring grooves on the mandrel in the area of the part projecting from the rivet flange, in such a manner that on this part of the mandrel there are no parts going beyond its stem diameter. When producing the blind-riveting assembly in question, a mandrel of this type can still be pushed into the rivent sleeve after forming the ring grooves.
The above-described developments show that the specialist field has long had the idea of improving the blind-riveting assembly from the point of view of its gripping surface for the setting tool. Now it should be borne in mind with regard to the technique of ring grooves on the mandrel that ring grooves of this type may under certain circumstances degenerate into an unwanted breaking point below the predetermined tensile strength of the neck. The same risk also applies, moreover, to the above mentioned blind-riveting assembly commonly used today, which possesses a smooth-surfaced nail-like mandrel and which, precisely because of this smooth surface, has to be gripped with a high contact pressure by the setting tool on the part projecting from the rivet head flange. Setting tools are usually equipped with serrated jaws, and the serrations are frequently sharp edged, and, because of the high pressure they exert, occasionally produce notch marks on th mandrel which create breaking points where the tensile strength is below that of the neck, thus causing the mandrel to break in the wrong place, as a result of which the setting process is not reliably carried out.
The patents hereinbefore referred to all describe blind-riveting assemblies of what are called the "open type", that is to say where the mandrel stem passe right through the rivet, its pulling stem projecting from the head flange of the rivet and its rivet-setting head exposed at the other end where its base engages the tail end of the rivet. The problem of providing for the pulling of the mandrel as hereinbefore discussed involves the same considerations, however, in the case where the rivet is closed at its tail end, and the setting head of the mandrel is wholly enveloped in the body portion of the rivet.